Many applications of power integrated circuits require driving inductive loads such as relays and solenoids. Driving an inductive load necessitates appropriate circuitry to dissipate energy stored in the inductive load when the integrated driver circuit switches off. Circuit designers currently use several different methods to deal with this energy dissipation issue.
A common problem for circuit designers is illustrated in FIG. 1. In FIG. 1, a power MOSFET 12 has a source coupled to circuit ground and a drain coupled to an inductive load 14. Inductive load may be composed of a solenoid or a resistive type load with an inductance associated with a connecting cable. A short detection circuit 18 monitors load 14. In the event that load 14 becomes short circuited, (an automechanic shorting load 14 with a screwdriver may be one example) short detection circuit 18 manipulates a switch 20 to discharge the voltage on the gate of transistor 12 thus turning transistor 12 off. This "fast turn-off" triggers an inductive flyback at the drain terminal of power MOSFET 12. Inductive flyback occurs because current cannot change instantaneously through an inductor; therefore the output node voltage will increase to a large positive value very quickly when power MOSFET 12 turns off. This phenomena is well known and understood by those skilled in the art. The circuit configuration in FIG. 1 is referred to as a low side driver and is also well known by those skilled in the art. The method employed in FIG. 1 does not add any additional circuitry and allows power MOSFET 12 to handle the energy dissipated by load 14. When the inductive flyback voltage reaches the breakdown voltage of power MOSFET 12, MOSFET 12 begins to conduct in the reverse biased avalanche mode and will dissipate all the energy that was stored in the inductive load during the conduction cycle.
This method has a serious drawback; namely a reliability issue. When power MOSFET 12 turns off and experiences inductive flyback, it breaks down. When breakdown occurs the junction breaks down nonuniformly causing "hot spots" to appear at junctions in MOSFET 12 where current has concentrated in localized areas. High current concentrations in localized areas lead to transistor damage and long term reliability problems.
Another prior art method used to dissipate energy in switching is to place a clamping diode in parallel with power MOSFET 12 as shown in FIG. 2. In FIG. 2, a clamping diode 16 is placed in parallel with power MOSFET 12. Diode 16 is oriented such that the cathode is coupled to the drain of MOSFET 12 and the anode is coupled to circuit ground.
The circuit of FIG. 2 works similarly to the circuit of FIG. 1 with the exception occurring during MOSFET 12 switching. When switch 20 is closed the gate of MOSFET 12 is discharged through switch 20 and power MOSFET 12 will turn off thereby initiating the inductive flyback. Due to the inductive flyback, the output node voltage increases quickly; when this voltage reaches the breakdown voltage of clamping diode 16 (clamping diode 16 is designed to break down at a lower voltage than power MOSFET 12 breakdown) clamping diode 16 will break down and dissipate the energy stored in inductive load 14 during the conduction cycle.
A disadvantage of this method is that clamping diode 16 requires a significant amount of semiconductor die area for the circuit layout. This occurs because clamping diode 16 must dissipate a large amount of energy when power MOSFET 12 is switching high currents. Because of the large amount of energy, clamping diode 16 must occupy approximately the same die area as large power MOSFET 12. This alternative, therefore, has a negative impact, especially in multiple output devices, on making the power integrated circuit a cost-effective alternative to discrete power solutions.
As power integrated circuits switch larger currents and dissipate larger amounts of energy, better methods must be developed to provide switching reliability at a competitive cost.
It, accordingly, is an object of this invention to provide a circuit and method of dissipating the stored energy in inductive loads during switching that will protect the output device, conserve die area, and provide long term circuit reliability. Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification and drawings.